One day
The château courtyard and ramparts, free, first thing. The cathedral next door. Then the Passage Pommeraye and Place Royale on the way west.
Afternoon on the island for the machines — check the elephant's running times before you commit the day to it — and finish at the rings at dusk.
Two days
Day one as above. Day two: the memorial and the quay in the morning, then either the Musée d'Arts or, if it is shut, the Butte Sainte-Anne and Jules Verne.
Three and four days
Add Trentemoult by river shuttle and a walk up the Erdre — half a day that costs a transport ticket.
On a fourth day take a day trip: Guérande's salt pans, Saint-Nazaire's submarine base, or Clisson.
Do not plan museums for a Monday or a Tuesday without checking which day that one shuts.
Les Machines de l'Île
Château des Ducs de Bretagne
Passage Pommeraye
Trentemoult